<!DOCTYPE html>
<html>
<head>

<meta name="viewport" content="width=800">
<style>
html { font-size: 16px; }
body { width: 800px; margin: 0; overflow-y: hidden; }
</style>

</head>
<body>

<div style="width: 400px; float: left">
    <div style="width: 800px; float: left; font-size: 2.5rem">
        This text should be autosized to 40px computed font size (16 * 800/320), since the float:left causes this to be a new cluster, and it is 800px wide.<br>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </div>

    <div style="width: 600px; font-size: 1.875rem">
        This text should be autosized to 30px computed font-size (16 * 600/320), since its width:600px makes it wider than the lowest common ancestor of its enclosing cluster, hence it will itself become a cluster.<br>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </div>

    <div style="font-size: 1.25rem">
        This text should be autosized to 20px computed font-size (16 * 400 / 320), since it's not an autosizing cluster itself and its parent element is the enclosing autosizing cluster which is 400px wide.<br>
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </div>
</div>

</body>
</html>
